All contact sensors dropped out 2 nights ago and the system "rebuilt" during the day. This morning, my contact sensor activated light took about 5 seconds to activate, and the rest of the sensors were offline. Each came back online after opening and closing. I pulled up the logs and noticed that the single key fob left and arrived about every 5 minutes from 2 am onward. The contact sensors were reporting temperatures at very short intervals, too, thus indicating multiple drops during the night. Any ideas on this? I am running the latest update, and the ZigBee mesh has been rock solid until now. No changes or additions to anything else in the house that might be interfering. Is there something in the latest update that could be causing this?
Please list your Zigbee devices (specific models) as some are known to cause issues with a Zigbee network. Since mesh networks automatically can adjust themselves, what was working fine yesterday could be 'all meshed up' 
today.
Combo of Iris V1, Iris V2 and Samsung multi-sensors. All of the them were haywire. I have multiple ZigBee outlets throughout the house to strengthen the mesh and have had no problems until the last few days.
What types of Zigbee outlets are you using? Do you have any Zigbee light bulbs? What keyfob device? We really need to whole picture to see if anything stands out as a known, problematic device.
In you look in Live Logs (or Past Logs) do you see any abnormal activity, aside from the temp / contact reporting interval and presence sensors? My guess is those those were probably due to these devices reconnecting to the network and sending a status report. Seems like something in your network is causing an issue... 
No light bulbs. Combo of Iris V1, Iris V2 and Sylvania outlets. The Sylvania outlets are spread out to enhance the mesh. Keyfob is Iris V1 - 4 button. The timed events on the outlets seem to be working fine......the issues seems to be with the battery powered "smaller" sensors.
There are no new wi-fi devices that might be interfering. I'll check the logs when I get home to see how it has been acting during the day today.
The only change has been the hub update. It was performed Monday afternoon, and this issue started Monday night. Reboot did not help.
I do not have experience with the Sylvania Outlets personally. I do know that the Sylvania Zigbee bulbs are not reported to be good zigbee repeaters. @mike.maxwell might know more about whether or not the Sylvania outlets are considered 'good' or 'bad' repeaters. Those would be the most suspicious devices in your mesh that you've mentioned thus far.
Thanks for the input. So why would the outlets continue to work and why have they worked fine in the mesh so far? You are correct.....something has changed to cause this. The question is: Inside or outside factor? I have a very small house in a very rural area, so it's pretty easy to run down the potential external factors.
One hypothesis is that these sensors were directly connected to the HE hub...but after the reboot for the firmware update, some of them started passing data through the repeaters....
Or, one of the repeaters is going bad?
If it is still happening, perhaps Hubitat Support could take a look and help identify what the problem is?
I would try adding a couple peanut plugs. I have 3 of the Sylvania outlets and they never have dropped off the mesh, but they don’t seem to be good routers. I ended up adding Samsung and peanut outlets to my setup and placing the Sylvania outlets on the edge of the network where they seem to not be causing harm.
3 of the sensors and the fob are within a few feet of the hub. Surely some of them would be connected directly to it instead of repeating. The issue is the whole network of sensors dropping out. A single bad repeater shouldn't cause that.....at least it probably shouldn't.
I have a light that is 1 foot from the hub and is routing through an outlet in my bedroom upstairs. I would unplug the outlets, if you can, and see if the problem goes away.
I have plenty of the Iris V1 outlets. Are they good repeaters?
In my limited experience, Iris V1 outlets and sensors are problematic. Adding 2 outlets and one sensor made my zigbee mesh unstable, so I removed them. There are some Hubitat users who keep their Iris V1 devices on a separate HE hub from the rest of their devices.
I am using both V1 and V2 of contact, motion, buttons, and outlets without any issue. I had problems when I had Osram lamps on the same hub, but I moved them to their own hub and changed the channel on my main hub to 25. My iris V1 seemed to like channel 25, might have had something to do with WiFi interference, but V1 and V2 are living in harmony, along with Samsung devices.
